This invention broadly relates to television signal processing and, more particularly, to a combined video clamp and self-balancing inverter circuit for performing the DC coupling of a video signal between source and load and for providing substantially perfect inversion of any portion of a composite video signal. Accordingly, the invention has general utility for all television applications which require establishment of a DC reference level and/or inversion or reinversion of selected portions of a composite video signal.
Clamping circuits, per se, are well known in the television art and in the communications field in general. It is also well known that keyed clamps provide the most satisfactory means of establishing and maintaining a constant DC reference potential for a video signal. Such clamps eliminate the temperature sensitivity problems associated with DC coupled systems, and, as well, avoid the DC level shifts and poor low frequency performance associated with AC coupled video systems.
Inversion of selected portions of a video signal is also generally known and utilized in video equipment such as processing amplifiers, transmitters, and studio cameras. With the development of subscription television systems, encoding techniques have evolved which include selective inversion of the active video signal components of a standard television signal, e.g. on the basis of fields, lines, or groups of lines thereof. Typically, in such prior art systems, the phase inverting characteristics of certain transistor configurations, e.g. common emitter or common source, are employed to effect signal inversion at the encoder, and reinversion thereof at the decoder. Frequently, both inverted and non-inverted signals are obtained from separate output terminals of a single transistor stage, e.g. the collector and emitter. However, because such arrangements impart a different gain to the inverted and non-inverted signals, reinversion, or reconstitution, of the original video information is necessarily imperfect, producing distortion which degrades the quality of the reproduced images. These deficiencies are especially aggravated in those systems where inversion of whole fields is involved.
There is no known system nor technique, other than the present invention, which is capable of maintaining a constant balance between inverted and non-inverted signals, during both encoding and decoding, so that the inverted video information is reinverted exactly to its original form and characteristic.